1. Field of the Invention
This invention relates to a process using chemical vapor deposition for replicating, that is, making close copies of the finish and figure of preshaped structures. The invention has particular utility in the rapid fabrication of highly polished optics with only minor polishing being required on the replica to obtain a final product, and with the original substrate or mandrel being reusable. The invention also has utility in the rapid fabrication of such products as ceramic engine components and ceramic tubes.
2. Description of the Prior Art
In the field of optics, light detection and ranging (LIDAR) has come to be recognized as an important diagnostic tool for remote measurement of a diversity of atmospheric parameters such as minor species concentrations, pressure, temperature, and water vapor profiles, aerosol cloud distributions, and wind fields. LIDAR techniques such as measurement of back scattered signals, differential absorption, and Doppler shifts have been used to obtain information about the earth's atmosphere.
The performance of a LIDAR system depends upon the optical configuration of its receiving telescope. Often, due to space limitations such as in a shuttle borne LIDAR system, the length of the telescope is fixed. Therefore, the optical designer must select a particular shape and optics speed of the mirrors to maximize the throughput of the telescope. The most critical element in the receiving telescope is the primary mirror because of its size, weight, fabrication cost, and thermal exposure to the outside world. Since the received signal is directly proportional to the area of the primary mirror, it is important to use as large a primary mirror as feasible to obtain reasonable signal levels for accurate measurement. This is particularly true when a space-borne LIDAR system is used to measure wind profiles in the troposphere on a global basis.
The conventional techniques employed in the prior art for fabricating large (.gtoreq.1.0 meter diameter) mirrors are quite slow and time consuming. Several months to years are required to fabricate a large mirror from ultra low expansion silica glass or Zerodur, a product commercially available from Schott Glass Technologies, Inc., 400 York Avenue, Duryea,
18462. Since a number of space-based LIDAR systems are planned for the future, considerable attention is currently being given to the development of techniques for the rapid and economic production of large, high performance mirrors.
A spin casting technique has been proposed to fabricate 1.2 meter and 3.5 meter diameter glass mirror blanks containing lightweight honeycomb cells. Although this technique is relatively faster than the conventional mirror fabrication methods and produces lightweight mirrors, .the weight of these mirrors is still an order of magnitude more than permissible for many space applications. Further, the spin-casting technique is unsuitable for fabricating large mirrors of advanced ceramics such as silicon carbide (SiC), titanium diboride (TiB.sub.2), and boron carbide (B.sub.4 C) that have high melting points. These latter materials have properties superior to those of glass for large lightweight optics.
Other techniques involving the casting of fiber reinforced composites containing epoxy and plastics and the stretching of membranes over appropriate substrates are also currently under investigation.
A process is disclosed in application for patent bearing Serial No. 389,248, filed Aug. 3, 1989 by J. T. Goela, M. A. Pickering and R. L. Taylor and assigned to the assignee of the present invention, for fabricating, by vapor deposition, lightweight structures out of refractory materials. The methods and lightweight structures disclosed in that application, which application by reference is incorporated herein, involve a core made of graphite to define the shape and size of each structure. The core is coated with an appropriate deposit, such as SiC or silicon (Si), to give the structure strength and stiffness and for bonding thereof to another surface, for example, the surface of a substrate comprising the faceplate of a mirror being fabricated.
In the fabrication of mirrors, it has been proposed, as disclosed in the above mentioned application for patent, to use graphite to form a substrate or mandrel for replicating on a SiC faceplate. One side of the mandrel is optically fabricated, either as flat or as a convex spherical shape. The other side of the mandrel is lapped flat. The lapped side of the mandrel is bonded by means of pillars and graphite cement to a baffle plate in a vapor deposition reactor. The mandrel is then coated with multiple coats of a suspension of carbon in solvent, following which the surface of the mandrel is buffed or polished to make it as shiny as possible without significantly altering its figure. Deposition of silicon carbide on the mandrel is then effected. Without separating the faceplate from the mandrel, the exposed silicon carbide surface may be etched with hot potassium hydroxide (KOH) to improve bonding of graphite to silicon carbide. A lightweight structure core of graphite is then fabricated and bonded with graphite cement to the silicon carbide surface of the mandrel. Silicon carbide is then chemically vapor deposited to enclose the core following which the baffle plate is separated from the baffle pillars. Controlled edging may be performed to remove excess silicon carbide. Using a blade, the interface between the graphite mandrel and the silicon carbide faceplate may then be opened to recover the silicon carbide mirror faceplate.
While this process disclosed in the aforementioned application for patent has advanced the development of lightweight rapid optics fabrication technology, the final product produced by the process is a mirror blank that is not of the desired high optical quality, that is, high finish, required for space-based LIDAR systems. A reason for this is that graphite which is used for the mandrel does not take a high polish. Additionally, there is a substantial difference in the coefficients of thermal expansion (CTE) between graphite and silicon carbide. Graphite deforms substantially more than silicon carbide at the high temperatures (about 1300.degree. C.) at which deposition of silicon carbide in a chemical vapor deposition reactor takes place. As a result, replication of the ambient or room temperature figure of a graphite substrate or mandrel by chemical vapor deposition of silicon carbide is not achievable in practice. Compensation for the difference in coefficients of thermal expansion is exceedingly difficult to effect.
Thus, there is a need and a demand to provide a rapid fabrication process to replicate with high optical quality the finish and figure of preshaped optical and other structures. The present invention was devised to fill the technological gap that has existed in the art in these respects.